You can buy canned blueberry filling that's already thickened, but fresh tastes so much better. Blueberry pies are traditionally thickened with cornstarch or flour, but instant tapioca is a suitable alternative. If you're using frozen berries or the berries are extremely juicy, combine them with sugar and allow them to rest for 30 minutes. Drain off the excess juice and stir in the thickener.
Cornstarch, like flour, is a grain-based starch derived from corn. It's inexpensive, widely available and simple to use. Because cornstarch thickens almost instantly when it comes in contact with hot liquids, you must mix it with cold liquids to make a slurry before adding it to a sauce. When making a blueberry pie, you can combine it with the sugar and fruit instead. Instant tapioca comes from the manioc root. You might have to search a bit more to find instant tapioca, but it's often a good choice for pie fillings because it produces a glossy, translucent fruit pie and has almost no taste.
The Plus Side
You can replace cornstarch with instant tapioca in a blueberry pie with several advantages. Cornstarch can sometimes cause a murky, clouded appearance in a berry pie. Tapioca always produces clear, glossy results. Cornstarch can't stand up to long periods of heat and it breaks down if frozen. If you want to freeze baked or unbaked pies, instant tapioca is the thickener of choice.
The Down Side
The main disadvantage of using instant tapioca to thicken a blueberry pie is that the granules never completely dissolve, so you have tiny bits of tapioca in the filling. Toss the tapioca in a blender or food processor to eliminate this problem or use tapioca starch instead. Another point to remember is that tapioca needs high heat to set up. Bake the blueberry pie at 400 degrees Fahrenheit for the first 10 to 20 minutes to set the filling. After that, you can turn the heat down to 375 F. Cover the edges of the piecrust with aluminum foil if they're browning too quickly.
Making the Switch
To substitute instant tapioca for cornstarch in your blueberry pie, use two parts tapioca for every one part cornstarch called for in the recipe. Combine the instant tapioca with the sugar and then stir in the fruit and any other ingredients, such as cinnamon or lemon juice. You can cover the pie with plastic wrap and freeze it at this point for later baking or proceed directly to the oven. Make sure the oven is completely preheated before putting the pie in it, and bake until the juices are thick and bubbly. Sometimes a pie looks like it's done because the crust is golden, while the filling actually needs a bit more time to thicken completely.